The field of this invention is that of tools and methods used for the cleaning of pipelines, especially the long, extended reach pipelines in offshore areas. As hot production crude is produced from the reservoirs below the ocean floor up to the wellhead equipment at the ocean floor and then through pipelines along the ocean floor, it is cooled by the relatively cool temperature of the ocean water. In deep water, the temperature can be a cold as 34 degrees Fahrenheit.
A characteristic common to a majority of the oil produced is that there is a paraffin component to the oil which will deposit on the walls of the pipeline and become a solid at temperatures well above the 34 degrees Fahrenheit. In fact, some of the paraffins become solid at temperatures above 70 degrees Fahrenheit, and so can be deposited or plated on the internal diameters of the pipelines at any expected ambient temperature. The process is similar to discussions of blocking of the arteries of a human being, with a thicker coating building up with time. Some of the pipelines have become so plugged that more than 90% of the flow area is blocked with waxes or paraffins. In addition to slowing production, the coatings are a hazard as they can frequently lead to complete blockages of flow in the pipelines.
Typically, the wall becomes layered with paraffin as the temperature of the oil goes below the solidification temperature of the paraffins in the produced fluids. The paraffins act as a sort of insulation to the flowing fluids in the pipeline, allowing it to maintain a higher temperature for a greater distance. The effect of this is to extend the distance along the pipeline which the paraffin is plating onto the internal diameter of the pipeline.
A common cure for the paraffin plating out on the internal diameter of the pipeline is to insert a pig into the flow stream and let the pig remove some of the paraffin. A pig is typically a cylindrical or spherical tool which will brush against the internal diameter of the pipeline in hopes of removing the deposited paraffins. In pipelines with a high incidence of deposited paraffins, a regular maintenance of pigs is normally prescribed as a preventative against pipeline blockages.
One problem with the pigs is that the deposited paraffins are relatively soft and contain a lot of oil. To some extent, the pigs actually compress the paraffins against the wall and squeeze the oil out without removing the paraffins, leaving a harder and stronger paraffin remaining.
A second problem is that when the paraffin layer on the internal diameter of the pipe is too thick, sloughing off may occur. If the paraffin starts to separate from the wall and continues, the pig begins to literally plow a block of paraffin ahead of itself. The pig will continue driving more and more paraffin off the wall of the pipeline until the pressure of the pipeline will no longer be able to move the mass. At that time, you have a full pipeline blockage, which cannot be moved by pressure from either end.
At that time, the plug of paraffin must be removed by chemicals. Characteristically, the way chemicals are deployed to the location of the blockage is to use a string of coiled pipe or coiled tubing which is unreeled into the pipeline to provide a circulation path for the circulation of chemicals. As the end of the coiled pipe reaches the location of the blockage, the chemicals are circulated either out the coiled tubing and back through the annulus outside of the coiled tubing and inside the pipeline, or the flow will be in the opposite direction.
Before the pipelines are completely blocked, wire rope pigs such as are described in U.S. Pat. No. 7,998,276 offer and improved means for removing the paraffins plated onto the inner walls of the pipelines by restraining the movement of the pig with a wire rope and allowing a differential to be built up across the pig to provide a jetting pressure. The jetting pressure can compliment or substitute for the chemicals for improved cleaning. The wire rope pig provides and internal valve which allows the flow through the pig to be bypassed through a vent to allow the pig to be pulled back without pulling all the pipeline fluids back with it.
All the methods described have required the stopping of the revenue producing production and allowing pumps to take over the control process. In addition to the loss of production, the pumps are expensive to rent and expensive to mobilize to the jobsite, especially when offshore.
A need has long existed for a method of remediating the wax or paraffin buildup using jetting pressure without the use of expensive chemicals, and now shutting the expensive production flow down to accomplish it.